• Dec 04, 2024
• News

Crane Overload Protective Devices

Crane overload protection facility is a safety protection device to prevent overloading of the crane, and an alarm will occur when the crane load longitudinally exceeds the rated value.

Introduction: The Need for Crane Overload Protective Devices

Crane overload protective devices play an important role in ensuring the safety and reliability of lifting operations. Overloading a crane can lead to equipment failure, structural damage, and even catastrophic accidents. These devices prevent cranes from lifting loads that exceed their rated capacity, thereby safeguarding personnel, equipment, and materials.

What Are Crane Overload Protective Devices?

Crane overload protective devices are safety mechanisms installed in cranes to monitor the load weight and halt operations if the load exceeds the crane's rated capacity. These devices are mandatory in many industries to comply with safety regulations, such as those outlined by the Occupational Safety and Health Administration (OSHA).

Purpose of Overload Protection

• Prevent Structural Damage: Protect crane components, including cables, hooks, and beams, from damage caused by excessive loads.
• Enhance Worker Safety: Minimize the risk of accidents due to equipment failure or tipped cranes.
• Improve Equipment Longevity: Reduce wear and tear on the crane, lowering maintenance costs and downtime.

Overview of Crane Overload Protective Devices

Crane overload protective devices are essential safety systems designed to prevent lifting operations from exceeding the crane's rated capacity. By continuously or intermittently measuring load, torque, or moment, these devices give operators timely feedback—visual, audible, or automatic shut-offs—to avoid dangerous overload conditions. The following sections describe the four main categories of overload protection, their operating principles, and typical applications.

1. Load Moment Indicators (LMIs)

Load Moment Indicators continuously monitor the crane's hoisting load in relation to boom length, angle, and counterweight settings, calculating the moment to ensure it remains within safe limits. Operators receive real-time visual displays—often backlit LCD screens—and audible alarms as the load approaches its maximum permissible moment. Advanced LMIs can even trigger automatic shutdown of hoisting or slewing functions when an overload threshold is breached, providing comprehensive protection on mobile cranes, and large overhead cranes.

2. Load Cells

Load cells are precision transducers installed in the hoist block or hook assembly that directly measure the weight of the load. These devices convert force into an electrical signal, which is fed into the crane's control system for real-time display and logging. Load cells offer high accuracy (often better than ±0.5%) and can integrate seamlessly with digital monitoring systems, making them ideal for applications where precise load data is critical—such as heavy-lift operations, alloy handling, and process-critical lifts in manufacturing plants.

3. Overload Limit Switches

Overload limit switches are mechanical or electromechanical devices set to trigger at a predetermined load threshold. When the hoist tension exceeds this preset limit, the switch opens the control circuit, instantly cutting power to the hoist motor. Their simplicity and low cost make them popular on smaller overhead cranes, jib cranes, and basic hoist systems. However, because they only provide a single cutoff point without graduated warnings, they are most effective when combined with other monitoring methods.

4. Torque Limiters

Torque limiters protect cranes that incorporate slewing or rotating components by measuring the twisting force transmitted through shafts or gearing. If the torque exceeds the safe design value—due to snagging, overweight loads, or sudden stops—the limiter disengages or slips, halting rotation to prevent gear damage and structural overload. Torque limiters are commonly found on rotating gantry cranes, tower cranes, and specialized ship-to-shore container cranes where rotary motion requires dedicated overload protection.

How Crane Overload Protective Devices Work

Crane overload protection systems safeguard both the equipment and personnel by preventing lifts that exceed a crane's safe capacity. These devices operate through a series of coordinated steps:

1. Load Detection

A built-in load sensor—often a strain gauge, load cell, or electronic load moment indicator—measures the force exerted by the lifted weight. This sensor continuously samples the load in real time, ensuring that the system always “knows” how heavy the hook is pulling.

2. Data Analysis

The sensor's output is fed into a control unit or onboard processor. Here, the measured load is compared against the crane's predefined rated capacity. The system factors in safety margins and dynamic effects such as acceleration or load swing to determine whether the lift remains within safe limits.

3. Operator Alerts

As the detected load approaches a set threshold (for example, 90 % of rated capacity), the protection device issues warnings. These may include flashing indicator lights on the control panel, audible beeps or buzzers, and messages on an operator display. Early alerts give the crane operator time to reduce the load or stop lifting before an overload occurs.

4. Automatic Protective Action

If the crane continues to lift beyond its capacity—even after warnings—the system intervenes. In many designs, an overload switch cuts power to the hoist motor or engages a mechanical brake. Some advanced systems will first slow the hoisting speed and then fully halt any upward motion, preventing the load from increasing further.

For example, a classic implementation is an electromechanical overload switch mounted on the hoist drum. When the tension in the wire rope exceeds a preset level, the switch trips and breaks the electrical circuit supplying the hoist motor. This immediate cut-off stops lifting and locks the brake, protecting the crane from structural damage and reducing the risk of accidents caused by dropped or uncontrolled loads.

Benefits of Using Overload Protective Devices

1. Compliance with Safety Standards

Regulatory bodies like OSHA and ISO mandate the use of overload protection in cranes to prevent workplace accidents. Compliance ensures legal safety requirements are met.

2. Enhanced Safety

Preventing overloads reduces the risk of accidents caused by equipment failure.

3. Cost Efficiency

Avoiding overload-induced damages minimizes repair costs and operational downtime. This contributes to long-term financial savings.

4. Operational Efficiency

These devices ensure that cranes operate within safe limits, enhancing their reliability and reducing the need for frequent maintenance.

Common Overload Risks in Crane Operations

Overload protection devices address specific risks, such as:

1. Overloading the Crane: Exceeding the weight capacity of the crane’s hoisting system.
2. Off-Center Loads: Uneven distribution of weight, leading to instability.
3. Dynamic Loading: Sudden movements that temporarily increase load forces.
4. Side Pulling: Applying lateral force to the crane, which can stress components.

Inspection and Maintenance of Overload Protection Devices

To ensure the reliability of overload protective devices, regular inspection and maintenance are essential. Recommendations include:

• Daily Visual Checks: Inspect for visible wear or damage.
• Periodic Calibration: Calibrate load cells and other measurement tools to maintain accuracy.
• Routine Testing: Conduct operational tests to ensure the device activates appropriately during overload conditions.

Failure to maintain these devices could result in equipment malfunctions, as highlighted in numerous safety studies.

Conclusion

Crane overload protective devices are indispensable for ensuring safe and efficient lifting operations. Yuantai cranes include a variety of safety features to make your operation safer.